1. Field of the Invention
This invention relates to a silicon devices/heatsinks stack assembly and a method to pull apart a faulty silicon device in said stack assembly retaining the silicon devices alignment during its replacement. In the following, we will consider that silicon devices are thyristors.
2. Description of the Related Art
In the field of electric power engineering and electric power transmission, a SVC (Static VAR Compensator) is used in the industrial and utility applications to increase power quality factors on the distribution and transmission lines. A key component of such a device is a thyristor valve. A thyristor valve is comprised of following components:                a stack of layered sets of heatsinks and thyristors, comprising:                    thyristors, which are silicon devices, allowing control of the current flow via the thyristor valve,            heatsinks, which are mechanical devices allowing removal of the heat from the thyristors, driven by conduction and switching losses, thus being able to operate the thyristor valve within a wide range of voltages and currents,                        auxiliary circuits as snubber circuits, gate drives, gate drive energy delivery systems allowing to reliably control the thyristor valve within specified application ranges.        
So said thyristor valve is built from the layered stack of thyristors packaged in the hockey puck (or disk) type of package and heatsinks Any other silicon devices packaged in this form factor can be utilized in the stack assembly, for example: rectifier diodes, bidirectional thyristors, IGBT's, triacs.
Such assemblies are also used in TCR (Thyristor Controlled Reactor) and TSC (Thyristor Switched Capacitor) valves for static VAR Compensators (SVC), in TCSC (Thyristor Controlled Series Capacitor) valves, and HVDC modules.
A thyristors/heatsinks stack assembly comprises multiple arrangements of thyristors disks and heatsinks devices in which each thyristor disk is separated from a next thyristor disk by a medium cooled heatsink device. Heatsinks devices may be composed of aluminium blocks with a special grid of flow channels, allowing a cooling medium (coolant, mostly a mixture of water and glycol) to flow through them. A thyristor disk is compressed between two heatsinks devices, thus having a close contact to the heatsink devices surface and the cooling medium flowing through it. Heat generated by thyristor disks is then removed by adjacent heatsink devices and transferred into the cooling medium and evacuated to heat exchangers, mounted outside of the thyristor valve.
The size of the stack assembly depends of the number of thyristor disks forming it. The number of stacked thyristor disks is determined by the thyristor valve electrical ratings: minimum, maximum, nominal voltage withstand, minimum, maximum, nominal thyristor valve current. Usually, there are between 3 and 30 thyristors in a stack assembly. The length of such a stack assembly can reach up to 2 meters.
A potential mechanical failure of thyristors/heatsinks stack assembly during maintenance procedures is identified as a potential problem.
Whenever there is a need to replace a faulty thyristor, the thyristors/heatsinks stack assembly has to be separated to accommodate removal and replacement of the faulty thyristor using spreaders, in breaking the alignment of the stack assembly. Lack of this alignment poses a potential threat for either a misalignment of the stack during reassembly, causing thyristor damage, or for a full collapse of the stack. This is especially true in the case of valve arrangements where the stack is mounted in a horizontal position.
As shown on FIG. 1, a conventional vertical thyristors/heatsinks stack assembly comprises an alternating stack of thyristor disks 10 and heatsink devices 11. The total number of stacked thyristors disks and heatsink devices can be up to 30 in one assembly. A properly assembled thyristors/heatsinks stack assembly has well aligned thyristors disks and heatsink devices. Typically, both thyristors disks and heatsink devices are equipped with centering holes 12. During the assembly a centering pin is placed in the two adjacent centering holes 12 allowing proper mechanical alignment, along an alignment path 13, between thyristor disks 10 and heatsink devices 11. However, maintaining the proper alignment becomes very challenging during a process of replacing any of the thyristor disks 10 in the assembled stack, as shown in FIG. 2, due to the fact that a standard spreader tool used during this process do not guarantee keeping in alignment remaining thyristor disks 10 and heatsink devices 11. So removal of a faulty thyristor disk may be accomplished by using an hydraulic or air actuated spreader tool to separate two heatsinks devices which are in contact with the faulty thyristor disk whilst retaining sufficient clamping on the other thyristor disks. On FIG. 2 a missing alignment path 14 is shown while a thyristor disk 10, which was positioned in the space shown by reference 15, is being replaced.
Alignment difficulties are even more pronounced during the assembly and replacement of thyristor disks in an horizontal thyristors/heatsinks stack assembly. The weight of the stack causes additional issue as thyristors/heatsinks stack assembly is being divided in two separate subassemblies. FIG. 3 show an horizontal thyristors/heatsinks stack assembly.
Until now, conventional methods and solutions of maintaining the alignment of the thyristors/heatsinks stack assembly do not provide a 100% assurance of retaining that alignment while a faulty thyristor is being replaced.
A prior art spreading method, used in a vertical stack configuration, relies on assumption that vertical configuration does not produce horizontal forces that could dislodge both subassemblies of the stack assembly. Due to the weight of the upper subassembly, there is a possibility of an horizontal shift due to forces exerted by the spreading tools. The skill of the technician greatly influences the final results of the alignment. Lack of which potentially could be a cause for damaged thyristor and faulty valve.
Such a prior art spreading method is represented on FIG. 4-5B. Reference 40 shows centering pin/thyristor centering hole mismatch. On each side of a faulty thyristor 43, a spreading tool 44 is used to pull apart the next heatsinks 11 to remove this faulty thyristor 43 and to replace it. Said spreading tool 44 comprises a first part 45 with an interior chamber, a second part 46 moving in said interior chamber to form an adjustable spreading actuator. A centering pin 41 and a corresponding centering hole 42 are represented on FIG. 5B which corresponds to detail A of FIG. 5A.
Such a prior art spreading method has the following deficiencies:                Used spreading actuators are very bulky. Therefore it is difficult to fit these actuators into the stack assembly. Spreading actuators are in special conflict with the stack assembly components. So thyristor replacement is very difficult and lengthy.        Spreading actuators in their active state always have slightly different force vectors on each side of the thyristor assembly. The result is out of the alignment centering pin, centering hole positions.        Alignment of the centering pins/holes is only possible after several trials and careful adjustments of both spreading actuators.        
A prior art spreading method, used in a horizontal stack configuration, relies on presence of conventional made to fit support spacers, furthering and stabilizing the thyristors/heatsinks stack assembly. Accuracy of the alignment depends greatly on the skills of technician performing this work and is prone to errors, both in the setup stage as well as during a faulty thyristor replacement process. Since both separated subassemblies of the stack are not connected, any unwanted movement will cause a loss of alignment. Furthermore, spreading tools used to separate both subassemblies of the stack assembly could potentially cause an unintended horizontal shift of the stack assembly, which in turn might lead to total loss of alignment on all stack components.
The above prior art solutions are expensive, unreliable and require at least two trained technicians to perform such a work. In case of possible failure to maintain the alignment, thyristors/heatsinks stack assembly has to be rebuilt, which would require an additional time and work to complete the task.
So a technical problem to be solved is the following one. Misalignment of the thyristors within the thyristors/heatsinks stack assembly could cause the damage to the thyristor, or could possibly be a cause for total collapse of the thyristors/heatsinks stack assembly, especially if the quantity of thyristors within the stack is increased. Moreover, during a thyristor replacement procedure, there must be provision to maintain the continuum of the alignment path of the thyristors/heatsinks stack assembly, even when a faulty thyristor is removed and the stack is divided into two separated subassemblies.
The purpose of the invention is to provide a silicon devices/heatsinks stack assembly and a method to pull apart a faulty silicon device in said stack assembly in retaining a full alignment of the silicon devices/heatsinks stack assembly during a silicon device replacement procedure, regardless of the silicon devices/heatsinks stack assembly orientation, which may be horizontal or vertical.